1) 55 countries grouped into
four leagues for each two-season period, based on ranking, so that
you're only playing against teams of a similar quality.2) Each league divided
into four divisions by a draw, so each team plays two or three
others, home and away.3) Promotion and
relegation between the leagues.4) Four divisional
winners of the top league go through to a final tournament (semis
and final) to become league champions at the end of the first
season.5) Meanwhile, after the
divisional games are finished, the European Championship
qualifying groups are played between March and November (top two
of each qualify).6) Then, the 16 nations
league divisional winners go into the European Championship
play-offs, apart from those that have already qualified.7) They get replaced by
the highest placed team below them in their league.

So, to qualify, you can either do it by the traditional
route, through the groups, or by winning one of four play-off
tournaments (semis and final for each of the four leagues). To get
to the play-offs, you have to either:

Key differences:i) All will be
competitive fixtures against teams of similar quality, with much
fewer opportunities for friendlies.ii) More than one route
to the finals of a major tournament.iii) An additional
trophy to play for in years when there is no major tournament.iv) The 16
lowest-ranked teams are guaranteed (at least) one place in the
European Championship.

It's September, October and November
2018 to complete the league (groups of three or four), so that's
four or six games each, either way it covers six match dates. The
teams in three-team groups will have two spare match dates for
friendlies (if they can find opponents!).

Then groups of
five or six for the Euro 2020 qualifiers, so eight or ten games
each, covering ten match dates (two each in March, June,
September, October and November 2019), though June is also
earmarked for the Nations League semi-finals and final
(potentially two teams playing four games in June, unless they're
in a five-team Euro group?).

Euro 2020 play-offs are in
March 2020, only three months before the tournament. So this might
be the only time that some teams can arrange friendlies, just
before major tournaments. Maybe there'll be less meetings between
European nations and those from other continents.